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Indicating Character Encoding and Language for HTTP Header Field Parametersgreenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/HTTPheader field parameterinternationalization
By default, message header field parameters in Hypertext Transfer Protocol (HTTP) messages
cannot carry characters outside the ISO-8859-1 character set. RFC 2231
defines an encoding mechanism for use in Multipurpose Internet Mail Extensions
(MIME) headers. This document specifies an encoding suitable for use in HTTP header fields that is compatible with a profile of the encoding defined in RFC 2231.
Distribution of this document is unlimited. Although this is not a work
item of the HTTPbis Working Group, comments should be sent to the
Hypertext Transfer Protocol (HTTP) mailing list at ietf-http-wg@w3.org,
which may be joined by sending a message with subject
"subscribe" to ietf-http-wg-request@w3.org.
Discussions of the HTTPbis Working Group are archived at
.
XML versions, latest edits and the issues list for this document
are available from .
A collection of test cases is available at .
Umbrella issue for editorial fixes/enhancements.
Add implementation report.
Try to be consistent with the terminology defined in RFC 6365.
Noted by James Manger: "Presumably RFC5987 (or its predecessors) decided it was highly unlikely
that any parameter names in use ended in "*" (though they are valid)
so it could redefine the syntax of values for such names." - add a note that
the notation indeed overloads parameter name syntax and clarify the use.
Noted by James Manger: "Curiously, RFC5987 disobeys the proposed recommendations for new parameters.
It allows
foo*=UTF-8''coll%C3%A8gues
but not
foo*="UTF-8''coll%C3%A8gues"
That might be ok with a parser that understands token, quoted-string, and RFC5987,
but presumably it will cause problems when RFC5987 processing is done after
a "standard httpbis parser" handles the token | quoted-string step.
" - add a note clarifying that this is indeed a shortcoming of the format, and what it means for implementations.
The document refers normatively to RFC 2616. Should it continue to do so,
or should we wait for HTTPbis? This may affect edge case in the ABNF, such
as the definition of linear white space or the characters allowed in
"token".
By default, message header field parameters in HTTP () messages
cannot carry characters outside the ISO-8859-1 character set (). RFC 2231
() defines an encoding mechanism for use in MIME headers.
This document specifies an encoding suitable for use in HTTP header fields that is compatible with a profile of the encoding defined in RFC 2231.
This document obsoletes and
moves it to "historic" status; the changes are summarized
in .
Note: in the remainder of this document, RFC 2231 is only referenced
for the purpose of explaining the choice of features that were adopted; they
are therefore purely informative.
Note: this encoding does not apply to message payloads
transmitted over HTTP, such as when using the media type "multipart/form-data"
().
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document
are to be interpreted as described in .
This specification uses the ABNF (Augmented Backus-Naur Form) notation defined in
. The following core rules are included by
reference, as defined in :
ALPHA (letters), DIGIT (decimal 0-9), HEXDIG (hexadecimal 0-9/A-F/a-f), and
LWSP (linear whitespace).
Note that this specification uses the term "character set" for consistency
with other IETF specifications such as RFC 2277 (see ). A more accurate term would be "character
encoding" (a mapping of code points to octet sequences).
RFC 2231 defines several extensions to MIME. The sections below discuss
if and how they apply to HTTP header fields.
In short:
Parameter Continuations aren't needed (),Character Set and Language Information are useful, therefore a simple subset
is specified (), andLanguage Specifications in Encoded Words aren't needed (). defines a mechanism that
deals with the length limitations that apply to MIME headers. These
limitations do not apply to HTTP ().
Thus, parameter continuations are not part of the
encoding defined by this specification.
specifies how to embed
language information into parameter values, and also how to encode
non-ASCII characters, dealing with restrictions both in MIME and HTTP
header parameters.
However, RFC 2231 does not specify a mandatory-to-implement character set,
making it hard for senders to decide which character set to use.
Thus, recipients implementing this specification &MUST; support the
"UTF-8" character set .
Furthermore, RFC 2231 allows the character set information to be left out.
The encoding defined by this specification does not allow that.
The syntax for parameters is defined in
(with RFC 2616 implied LWS translated to RFC 5234 LWSP):
parameter = attribute LWSP "=" LWSP value
attribute = token
value = token / quoted-string
quoted-string = <quoted-string, defined in >
token = <token, defined in >
In order to include character set and language information, this
specification modifies the RFC 2616 grammar to be:
parameter = reg-parameter / ext-parameter
reg-parameter = parmname LWSP "=" LWSP value
ext-parameter = parmname "*" LWSP "=" LWSP ext-value
parmname = 1*attr-char
ext-value = charset "'" [ language ] "'" value-chars
; like RFC 2231's <extended-initial-value>
; (see )
charset = "UTF-8" / mime-charset
mime-charset = 1*mime-charsetc
mime-charsetc = ALPHA / DIGIT
/ "!" / "#" / "$" / "%" / "&"
/ "+" / "-" / "^" / "_" / "`"
/ "{" / "}" / "~"
; as <mime-charset> in
; except that the single quote is not included
; &SHOULD; be registered in the IANA charset registry
language = <Language-Tag, defined in >
value-chars = *( pct-encoded / attr-char )
pct-encoded = "%" HEXDIG HEXDIG
; see
attr-char = ALPHA / DIGIT
/ "!" / "#" / "$" / "&" / "+" / "-" / "."
/ "^" / "_" / "`" / "|" / "~"
; token except ( "*" / "'" / "%" )
Thus, a parameter is either a regular parameter (reg-parameter), as previously
defined in , or an extended
parameter (ext-parameter).
Extended parameters are those where the left-hand side of the assignment
ends with an asterisk character.
The value part of an extended parameter (ext-value) is a token that consists
of three parts: the &REQUIRED; character set name (charset), the &OPTIONAL;
language information (language), and a character sequence representing the
actual value (value-chars), separated by single quote
characters. Note that both character set names and
language tags are restricted to the US-ASCII character set, and are matched
case-insensitively (see and
).
Inside the value part, characters not contained in attr-char are
encoded into an octet sequence using the specified character set. That octet
sequence is then percent-encoded as specified in .
Producers &MUST; use the "UTF-8" () character set.
Extension character sets (mime-charset) are reserved for future use.
Note: recipients should be prepared to handle encoding
errors, such as malformed or incomplete percent escape sequences, or
non-decodable octet sequences, in a robust manner. This specification
does not mandate any specific behavior, for instance, the following
strategies are all acceptable:
ignoring the parameter,stripping a non-decodable octet sequence,substituting a non-decodable octet sequence by a replacement
character, such as the Unicode character U+FFFD (Replacement Character).Note: the RFC 2616 token production ()
differs from the production used in RFC 2231 (imported from )
in that curly braces ("{" and "}") are excluded. Thus, these two
characters are excluded from the attr-char production as well.
Note: the <mime-charset> ABNF defined here differs from
the one in in that it does
not allow the single quote character (see also RFC Errata ID 1912 ). In practice, no character set names
using that character have been registered at the time of this writing.
Note: did require support for ISO-8859-1,
too; for compatibility with legacy code, recipients are encouraged to
support this encoding as well.
Non-extended notation, using "token":
foo: bar; title=Economy
Non-extended notation, using "quoted-string":
foo: bar; title="US-$ rates"
Extended notation, using the Unicode character U+00A3 (POUND SIGN):
foo: bar; title*=utf-8'en'%C2%A3%20rates
Note: the Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, then percent-encoded. Also, note
that the space character was encoded as %20, as it is not contained in
attr-char.
Extended notation, using the Unicode characters U+00A3 (POUND SIGN)
and U+20AC (EURO SIGN):
foo: bar; title*=UTF-8''%c2%a3%20and%20%e2%82%ac%20rates
Note: the Unicode pound sign character U+00A3 was encoded into the octet sequence C2 A3 using the UTF-8 character encoding, then percent-encoded. Likewise,
the Unicode euro sign character U+20AC was encoded into the octet
sequence E2 82 AC, then percent-encoded. Also note that HEXDIG allows
both lowercase and uppercase characters, so recipients must understand
both, and that the language information is optional, while the character
set is not.
extends the encoding
defined in to also support language specification
in encoded words. Although the HTTP/1.1 specification does refer to RFC 2047
(),
it's not clear to which header field exactly it applies, and whether it is
implemented in practice (see
for details).
Thus, this specification does not include this feature.
Specifications of HTTP header fields that use the extensions defined
in ought to clearly
state that. A simple way to achieve this is to normatively reference
this specification, and to include the ext-value
production into the ABNF for that header field.
For instance:
foo-header = "foo" LWSP ":" LWSP token ";" LWSP title-param
title-param = "title" LWSP "=" LWSP value
/ "title*" LWSP "=" LWSP ext-value
ext-value = <see RFC 5987, >
Note: The Parameter Value Continuation feature defined in
makes it impossible to
have multiple instances of extended parameters with identical parmname components,
as the processing of continuations would become ambiguous. Thus, specifications
using this extension are advised to disallow this case for compatibility
with RFC 2231.
requires that protocol
elements containing human-readable text are able to carry language information. Thus, the ext-value
production ought to be always used when the parameter value is of textual
nature and its language is known.
Furthermore, the extension ought to also be used whenever the parameter value
needs to carry characters not present in the US-ASCII ()
character set (note that it would be unacceptable to define a new parameter that
would be restricted to a subset of the Unicode character set).
Header field specifications need to define whether multiple
instances of parameters with identical parmname components are allowed, and
how they should be processed. This specification suggests that a parameter using the
extended syntax takes precedence. This would allow producers to use both
formats without breaking recipients that do not understand the extended syntax
yet.
Example:
foo: bar; title="EURO exchange rates";
title*=utf-8''%e2%82%ac%20exchange%20rates
In this case, the sender provides an ASCII version of the title
for legacy recipients, but also includes an internationalized version for
recipients understanding this specification -- the latter obviously
ought to prefer the new syntax over the old one.Note: at the time of this writing, many implementations failed
to ignore the form they do not understand, or prioritize the ASCII form
although the extended syntax was present.
The format described in this document makes it possible to transport
non-ASCII characters, and thus enables character "spoofing" scenarios,
in which a displayed value appears to be something other than it is.
Furthermore, there are known attack scenarios relating to decoding UTF-8.
See for more information on
both topics.
In addition, the extension specified in this document makes it possible
to transport multiple language variants for a single parameter, and such use might allow spoofing attacks, where
different language versions of the same parameter are not equivalent.
Whether this attack is useful as an attack depends on the parameter
specified.
Thanks to Martin Duerst and Frank Ellermann for help figuring out ABNF details,
to Graham Klyne and Alexey Melnikov for general review, to
Chris Newman for pointing out an RFC 2231 incompatibility, and to
Benjamin Carlyle, Roar Lauritzsen, and Eric Lawrence for implementer's feedback.
Key words for use in RFCs to Indicate Requirement LevelsHarvard Universitysob@harvard.eduHypertext Transfer Protocol -- HTTP/1.1University of California, Irvinefielding@ics.uci.eduW3Cjg@w3.orgCompaq Computer Corporationmogul@wrl.dec.comMIT Laboratory for Computer Sciencefrystyk@w3.orgXerox Corporationmasinter@parc.xerox.comMicrosoft Corporationpaulle@microsoft.comW3Ctimbl@w3.orgIANA Charset Registration ProceduresUTF-8, a transformation format of ISO 10646Alis Technologiesfyergeau@alis.comUniform Resource Identifier (URI): Generic SyntaxWorld Wide Web Consortiumtimbl@w3.orghttp://www.w3.org/People/Berners-Lee/Day Softwarefielding@gbiv.comhttp://roy.gbiv.com/Adobe Systems IncorporatedLMM@acm.orghttp://larry.masinter.net/Tags for Identifying LanguagesLab126addison@inter-locale.comGooglemark.davis@google.comAugmented BNF for Syntax Specifications: ABNFBrandenburg InternetWorking+1.408.246.8253dcrocker@bbiw.netTHUS plc.paul.overell@thus.netCoded Character Set -- 7-bit American Standard Code for Information InterchangeAmerican National Standards InstituteErrata ID 1912, RFC 2978RFC ErrataInformation technology -- 8-bit single-byte coded graphic character sets -- Part 1: Latin alphabet No. 1International Organization for StandardizationMultipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message BodiesInnosoft International, Inc.ned@innosoft.comFirst Virtual Holdingsnsb@nsb.fv.comMIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII TextUniversity of Tennesseemoore@cs.utk.eduMIME Parameter Value and Encoded Word Extensions: Character Sets, Languages, and ContinuationsInnosoft International, Inc.ned.freed@innosoft.comUniversity of Tennesseemoore@cs.utk.eduIETF Policy on Character Sets and LanguagesUNINETTHarald.T.Alvestrand@uninett.noReturning Values from Forms: multipart/form-dataXerox Palo Alto Research Centermasinter@parc.xerox.comCharacter Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parametersgreenbytes GmbHHafenweg 16MuensterNW48155Germanyjulian.reschke@greenbytes.dehttp://greenbytes.de/tech/webdav/Web Linkingmnot at mnot.netUse of the Content-Disposition Header Field
in the Hypertext Transfer Protocol (HTTP)greenbytes GmbHjulian.reschke@greenbytes.de
This section summarizes the changes compared to :
The document title was changed to "Indicating Character Encoding and Language for HTTP Header Field Parameters".
The requirement to support the "ISO-8859-1" encoding was removed.
The encoding defined in this document currently is used for two different
HTTP header fields:
"Content-Disposition", defined in , and
"Link", defined in .
As the encoding is a profile/clarification of the one defined in
in 1997, many user agents already supported it for
use in "Content-Disposition" when got published.
Since the publication of , two more popular desktop
user agents have added support for this encoding; see
for details. At this time, only one major desktop user agent (Safari)
does not support it.
Note that the implementation in Internet Explorer 9 does not support the
ISO-8859-1 encoding; this document revision acknowledges that UTF-8 is
sufficient for expressing all code points, and removes the requirement
to support ISO-8859-1.
The "Link" header field, on the other hand, was only recently specified
in . At the time of this writing, no User Agent
supported the "title*" parameter, using the encoding defined by this document,
but implementation for Firefox was already in progress (see
).
Only editorial changes for the purpose of starting the revision process
(obs5987).
Resolved issues "iso-8859-1" and "title" (title simplified).
Added and resolved issue "historic5987".
Added issues "httpbis", "parmsyntax",
"terminology" and "valuesyntax".
Closed issue "impls".